linux/Documentation/RCU/rculist_nulls.txt

Using hlist_nulls to protect read-mostly linked lists and
objects using SLAB_DESTROY_BY_RCU allocations.

Please read the basics in Documentation/RCU/listRCU.txt

Using special makers (called 'nulls') is a convenient way
to solve following problem :

A typical RCU linked list managing objects which are
allocated with SLAB_DESTROY_BY_RCU kmem_cache can
use following algos :

1) Lookup algo
--------------
rcu_read_lock()
begin:
obj = lockless_lookup(key);
if (obj) {
  if (!try_get_ref(obj)) // might fail for free objects
    goto begin;
  /*
   * Because a writer could delete object, and a writer could
   * reuse these object before the RCU grace period, we
   * must check key after getting the reference on object
   */
  if (obj->key != key) { // not the object we expected
     put_ref(obj);
     goto begin;
   }
}
rcu_read_unlock();

Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
but a version with an additional memory barrier (smp_rmb())

lockless_lookup(key)
{
   struct hlist_node *node, *next;
   for (pos = rcu_dereference((head)->first);
          pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
          ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
          pos = rcu_dereference(next))
      if (obj->key == key)
         return obj;
   return NULL;

And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() :

   struct hlist_node *node;
   for (pos = rcu_dereference((head)->first);
		pos && ({ prefetch(pos->next); 1; }) &&
		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
		pos = rcu_dereference(pos->next))
      if (obj->key == key)
         return obj;
   return NULL;
}

Quoting Corey Minyard :

"If the object is moved from one list to another list in-between the
 time the hash is calculated and the next field is accessed, and the
 object has moved to the end of a new list, the traversal will not
 complete properly on the list it should have, since the object will
 be on the end of the new list and there's not a way to tell it's on a
 new list and restart the list traversal.  I think that this can be
 solved by pre-fetching the "next" field (with proper barriers) before
 checking the key."

2) Insert algo :
----------------

We need to make sure a reader cannot read the new 'obj->obj_next' value
and previous value of 'obj->key'. Or else, an item could be deleted
from a chain, and inserted into another chain. If new chain was empty
before the move, 'next' pointer is NULL, and lockless reader can
not detect it missed following items in original chain.

/*
 * Please note that new inserts are done at the head of list,
 * not in the middle or end.
 */
obj = kmem_cache_alloc(...);
lock_chain(); // typically a spin_lock()
obj->key = key;
/*
 * we need to make sure obj->key is updated before obj->next
 * or obj->refcnt
 */
smp_wmb();
atomic_set(&obj->refcnt, 1);
hlist_add_head_rcu(&obj->obj_node, list);
unlock_chain(); // typically a spin_unlock()


3) Remove algo
--------------
Nothing special here, we can use a standard RCU hlist deletion.
But thanks to SLAB_DESTROY_BY_RCU, beware a deleted object can be reused
very very fast (before the end of RCU grace period)

if (put_last_reference_on(obj) {
   lock_chain(); // typically a spin_lock()
   hlist_del_init_rcu(&obj->obj_node);
   unlock_chain(); // typically a spin_unlock()
   kmem_cache_free(cachep, obj);
}



--------------------------------------------------------------------------
With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
and extra smp_wmb() in insert function.

For example, if we choose to store the slot number as the 'nulls'
end-of-list marker for each slot of the hash table, we can detect
a race (some writer did a delete and/or a move of an object
to another chain) checking the final 'nulls' value if
the lookup met the end of chain. If final 'nulls' value
is not the slot number, then we must restart the lookup at
the beginning. If the object was moved to the same chain,
then the reader doesn't care : It might eventually
scan the list again without harm.


1) lookup algo

 head = &table[slot];
 rcu_read_lock();
begin:
 hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
   if (obj->key == key) {
      if (!try_get_ref(obj)) // might fail for free objects
         goto begin;
      if (obj->key != key) { // not the object we expected
         put_ref(obj);
         goto begin;
      }
  goto out;
 }
/*
 * if the nulls value we got at the end of this lookup is
 * not the expected one, we must restart lookup.
 * We probably met an item that was moved to another chain.
 */
 if (get_nulls_value(node) != slot)
   goto begin;
 obj = NULL;

out:
 rcu_read_unlock();

2) Insert function :
--------------------

/*
 * Please note that new inserts are done at the head of list,
 * not in the middle or end.
 */
obj = kmem_cache_alloc(cachep);
lock_chain(); // typically a spin_lock()
obj->key = key;
/*
 * changes to obj->key must be visible before refcnt one
 */
smp_wmb();
atomic_set(&obj->refcnt, 1);
/*
 * insert obj in RCU way (readers might be traversing chain)
 */
hlist_nulls_add_head_rcu(&obj->obj_node, list);
unlock_chain(); // typically a spin_unlock()